The transition from drug use to an addicted state is signaled by marked escalation in drug intake and increased drug seeking during withdrawal. Our studies suggest that up-regulation in cAMP/PKA signaling pathways in nucleus accumbens (NAc) following chronic drug use directly contributes to this transition, possibly by differentially altering D1 and D2 receptor-mediated responses that regulate drug-taking and -seeking behaviors. To investigate this hypothesis, studies measure Dl and D2 receptor responsiveness before, during and after chronic cocaine self-administration in Low and High intake rats. Behavioral studies track sensitivity to Dl and D2 regulation of locomotion (unconditioned responses) and relapse to cocaine seeking (conditioned responses) in relation to both time and individual propensity for escalation. Parallel biochemical studies track changes in downstream cAMP-dependent protein phosphorylation and related signaling proteins before, during and after chronic cocaine self-administration, for comparison in Low and High intake rats and their yoked partners. The functional consequence of up-regulation in cAMP/PKA signaling is studied in three anatomically distinct models of cAMP/PKA up-regulation. The first model utilizes cholera toxin microinfusion in NAc core and shell subregions in rats. The other two models utilize inducible transgenic mice that over-express Gs proteins in either Dl/dynorphin- or D2/enkephalin-containing striatal neurons. These studies will test the relative contribution of cAMP/PKA up-regulation in distinct NAc subregions and specific striatal cell types to escalating cocaine intake, long-term relapse to cocaine seeking induced by drugs, cues, and stress. Studies also investigate the role of cAMP/PKA up-regulation on altered D 1 and D2 receptor responsiveness in locomotion and relapse, with downstream cAMP-dependent protein phosphorylation as a biochemical correlate. AMPA glutamate receptors in NAc may regulate addictive behavior via specific interactions with D1- and D2- regulated cAMP/PKA signaling pathways. These interactions are studied using viral-mediated over-expression of GluRl and GluR2 AMPA subunits in NAc neurons in assays of cocaine self-administration and relapse to cocaine seeking. Studies also test specific AMPA receptor interactions with D1 and D2 receptors in regulation of locomotion and relapse, and their dependence on cAMP/PKA signaling using a PKA insensitive GluR1 vector. Together, these studies combine relevant behavioral models with modem molecular techniques to investigate discrete neural and behavioral alterations that contribute to the addiction process